The present invention relates generally to coating systems and more specifically to the protection of silicon containing materials, such as ceramic matrix composites (CMC""s), in hot, oxidizing environments, such as the combustor or turbine portion of gas turbine engines.
Higher operating temperatures for gas turbine engines are continuously sought in order to improve their efficiency. However, as the operating temperatures increase, the high temperature durability of the components of the engine must also correspondingly increase. Materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material and/or as a reinforcing material, are currently being used for high temperature applications, such as in the components for the combustor and/or turbine sections of gas turbine engines.
Silicon carbide (SiC) has a tendency to oxidize into silica (SiO2) and CO2 at elevated temperatures in the presence of O2. Some additional silica may be formed due to the presence of some free silicon. While the silica formed can be an excellent diffusion barrier to prevent diffusion of O2, silica is itself subject to deterioration in the presence of water or vapors of water such as steam. Thus, while coating systems have been used to provide some protection for the SiC in high temperature environments that include silica, these systems have been ineffective or deficient in some aspect. Either the silica layer that has been provided is very thin and formed by decomposition of the substrate or inadequate protection of the silica layer from a hydrous atmosphere has been provided. When a coating system is utilized for protection of the underlying SiC in hot, oxidizing environments is formed by preoxidation of the SiC substrate, which is not a preferred method because of the length of time required to form a silica layer of sufficient thickness, there is still a tendency for void formation to occur, even when the silica is protected from deterioration from a hydrous atmosphere. The voids may appear in the thin interfacial silica scale formed by the transformation. The voids can also exist at the interfaces between the silica scale and the substrate. The voids are undesirable as they decrease the environmental protection provided by any external coatings. Not only do the voids break down the SiC, which can adversely affect the mechanical properties of the CMC composite, which are designed around the mechanical properties of its system components, but the voids can also provide a path of least resistance that permits the continued inward diffusion of oxygen to promote further deterioration of the SiC composite. The voids can aggregate during the course of operation at high temperatures and can reduce the life of the external coating by promoting spallation of the applied coating. Alternatively, when inadequate protection of the silica layer is provided, the silica layer quickly deteriorates by contact with a hydrous atmosphere such as may be encountered by turbine engines in marine or coastal environments leading to a breakdown of the protective silica layer by the formation of volatile SiO and silicon hydroxide (Si(OH)x) products. This can then result in oxidation of the underlying SiC and rapid deterioration of the substrate in a hydrous oxidizing environment as the SiC decomposes and as any SiO2 formed during the decomposition of the SiC also decomposes. Thus, any coating system applied to the silicon-containing material should provide environmental protection by inhibiting the degradation of the silicon containing material in a water-containing or steam-containing environment. Thus, a dense layer of silica is effective in protecting a silicon-containing substrate from oxidation only so long as the silica layer is not itself degraded in a hydrous environment, such as those in which gas turbines frequently operate.
Various coating systems are available for the protection of silicon containing materials, such as silicon carbide systems, from oxidation at high temperatures and degradation in a water-containing environment. One type of coating system is discussed in U.S. Pat. Nos. 6,129,954 to Spitsberg et al. and 5,869,146 to McCluskey et al., which disclose techniques for applying a mullite (3Al2O3.2SiO2) coating to a silicon-based ceramic substrate. The mullite coating is used as a thermal barrier coating (TBC) for the silicon-based ceramic substrate. The mullite coating can also serve as a bond coat for the subsequent application of an environmental barrier coating (EBC) such as yttria-stabilized zirconia (YSZ) as there is a mismatch between the coefficients of thermal expansion of YSZ and silicon carbide. However, mullite does not provide adequate protection in high temperature environments containing water vapors because mullite has, thermodynamically, significant silica activity due to the high concentration of SiO2 in mullite, and volatilizes at high-temperatures in the presence of water or water vapor. Another coating system is discussed in U.S. Pat. No. 5,985,470 to Spitsberg et al., which discloses a thermal/environmental barrier coating system for a silicon-based ceramic substrate. The coating system includes a layer of barium strontium aluminosilicate (BSAS), which serves as a bond coat for a ceramic topcoat. The ceramic topcoat can include a zirconia partially or fully stabilized with yttria (YSZ) and yttrium silicate.
Other coating systems have proposed protecting the silicon-containing substrate by providing environmental protection using barium aluminosilicate and variations of this material and intermediate layers between the barrier layer and the substrate to enhance adherence and prevent reactions between the barrier layer and the silicon-containing substrate. The intermediate layers can include a bond coat of silica formed by preoxidation of the silicon-containing substrate or a layer of silicon applied to the substrate followed by application of the intermediate layer by thermal spray. The intermediate layer may be applied without the formation of a bond coat.
Still other coating systems for protecting CMC""s against oxidation include the formation of a thin silica layer to the silicon containing material. To provide additional thermal and environmental protection, a mullite coating and a ceramic topcoat of YSZ can be applied to the silica layer. Still other coating systems can apply a layer of silicon onto the silicon-containing material to improve adhesion, followed by additional thermal and environmental barrier coatings that are then applied onto the silicon layer.
Current coating systems are designed for interface temperatures between the protective coating and the substrate of not more than about 2300xc2x0 F. What is needed is a protective coating system that can increase this interface temperature into the range of about 2400-2500xc2x0 F. as the temperatures of the turbine are increased by use of a coating sufficiently thick to improve the temperature capability of the coating system, while eliminating components that may experience incipient melting at these elevated temperatures. The protective coating system ideally forms a barrier to diffusion of oxygen to protect the silicon-containing substrate from degradation, yet is stable in high temperature environments containing water molecules and can be selectively applied. The coating system must be easy to apply to the desired thickness and be capable of maintaining its resistance to oxygen diffusion, in the presence of water molecules and at temperatures of up to 3000xc2x0 F. Most importantly, the material should be also improve the elevated temperature adherence of thermal barrier coatings such as yttria-stabilized zirconia to the silicon containing substrate by providing an intermediate coefficient of thermal expansion (CTE) so that the component to which it is applied can be used in hydrous environments that experience even higher temperatures, while having excellent environmental properties.
The present invention is directed to a coating system for an article used in a hot section of a gas turbine engine. The article includes a ceramic matrix composite or CMC substrate that is oxidizable or otherwise degradable at elevated temperatures of operation in a corrosive environment. The coating system is comprised of an inner layer of material that has a higher melting temperature than the CMC substrate and a low oxygen permeability, which is applied over the CMC substrate to protect the substrate from oxidation. The coating system also includes an intermediate layer that protects the inner layer from deterioration due to exposure to a hot corrosive hydrous environment while also providing thermal protection. The intermediate layer can consist of at least one material selected from the group consisting of mullite and barium strontium aluminosilicate (BSAS), yttria-stabilized zirconia (YSZ), alumina and mixtures thereof applied over the inner layer and an thermal barrier coating of YSZ applied over the intermediate layer.
The present invention is also directed to a second coating system for an article used in a hot section of a gas turbine engine. The second article includes a CMC substrate formed of a silicon containing material. The coating system includes an inner layer of material, such as silica having a low diffusivity of oxygen and a higher melting temperature than the substrate applied over the composite substrate. The second coating system comprises an intermediate layer that includes a first sublayer of mullite applied over the inner layer, a second sublayer of BSAS applied over the first sublayer, a third sublayer applied over the second sublayer and underlying an outermost thermal barrier coating. The third sublayer consists essentially of a thermal-insulating material and at least one material selected from the group consisting of mullite, alumina and alkaline earth metal aluminosilicates and an outer layer of YSZ.
An advantage of the present invention is that the coating system inner layer forms a protective, diffusion barrier layer over the CMC composite that prevents the diffusion of oxygen through to the CMC. This prevents the deterioration of any materials present in the CMC that are subject to degradation by oxidation at elevated temperatures. The coating system also increases the temperature capability of the coated component by providing a coating system that can withstand elevated interface temperatures and that eliminates components that may exhibit incipient melting at elevated temperatures. When the inner layer is a material that is subject to deterioration in a hydrous atmosphere, the intermediate and outer layers are applied to provide protection from the detrimental effects of the hydrous atmosphere while also providing environmental and thermal protection. As used herein, hydrous environment or atmosphere means an environment or atmosphere that contain water, water vapor or steam.
Another advantage of the present invention is that the coating system is layered in such a manner that adherence of an advanced thermal barrier material to a CMC substrate, typically one containing silicon, is improved. This is accomplished by matching the CTE""s of the layer so that cyclic stresses resulting from the temperature gradients due to the high temperatures are reduced sufficiently to provide at least satisfactory life without serious spallation problems.
Another advantage of the present invention is directly related to the ability of the to form a diffusion barrier to inhibit the diffusion of oxygen. Since oxygen cannot penetrate the diffusion barrier layer, the deterioration of the silicon-containing substrate with the accompanying formation of voids at the interface between the substrate and the inner layer or below the interface and within the substrate is prevented. As void formation is related to spalling of materials above the voids, particularly as the voids coalesce, the spalling of the layers is prevented.
Another advantage is that the inner layer is not prone to incipient melting and associated deterioration and can form a strong bond with most non-metallic, CMC engineering materials that can be used as structural components in the hot portions of gas turbine engines. The typical thermal barrier coatings that overly these structural components to improve the components"" thermal response are also capable of forming good bonds with the inner layer.
A further advantage of the present invention is that it permits the use of oxidizable composite materials such as SiC and SiN, which do not contain free silicon, as substrates in applications in which the temperature experienced can be above the melting point of silicon, thereby allowing the use of these materials in applications not previously considered. An additional advantage is that metal components currently used in turbine engines can be replaced with lightweight composite components to reduce the weight of the engine, thereby improving specific fuel performance.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.